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Paper No. 88-19
Presentation Time: 9:00 AM-5:30 PM
OUTSTANDING LOWER DEVONIAN MILANKOVITCH CYCLICITIES EXPOSED IN THE HUDSON VALLEY, NEW YORK STATE
The Devonian Period (417-365 mya) was a dynamic time during Earth’s history. While there exists a relatively good understanding of the events that occurred during this time as a whole, our knowledge of smaller intervals during this time contains gaps. One interval of interest is the Early Devonian Emsian Stage. Uncertainties on the ages of Devonian stage boundaries, such as the Emsian, are currently in the order of several millions of years and a cyclostratigraphic approach can help to improve the Devonian time scale. The NY Rte 199 section at Kingston, NY, exposes most of the Schoharie Fm. It corresponds to the upper portion of the Emsian, spanning about 6 Myr with relatively continuous deposition. The lithology consists of carbonates with interbedded shale, with various degrees of bioturbation (Zoophycos, etc.) and colors ranging from beige white, to brown or dark grey. The quality of most of the outcrop is so remarkable that the color variations permit recognition of Milankovitch cycles, with prominent bundles of light and dark beds. One type of cycle expression is represented by a succession of about six darker beds nested between lighter beds, which is interpreted as six precession cycles in one short eccentricity cycle (precession in the Devonian was probably about 18 ky). A pilot sampling (first 5 m out of the ˜40 m of the outcrop sampled every 2.5 cm and about 20 samples distributed more or less evenly along the remainder of the section) demonstrated that in this section the average magnetic susceptibility is about 6.10ˆ-8 m3/kg. Hysteresis measurements provide a high field susceptibility of about ˜5.10ˆ-8 m3/kg and most of the hysteresis loops are straight lines, with relatively small impact of ferromagnetic grains. The correlation between the high field susceptibility and the magnetic susceptibility is high (r= 0.91), while the correlation between the ferromagnetic susceptibility and the magnetic susceptibility is much lower (r=0.17). Thus, the ferromagnetic minerals have a low impact on the total magnetic susceptibility; its variability is driven by paramagnetic clay minerals. Importantly, despite being remagnetized (throughout the Appalachians, these Paleozoic rock sequences are all remagnetized during the Variscan-Alleghanian orogeny) the magnetic susceptibility reflects depositional information.